1. Field of the Invention
The present invention relates to a novel parabolic wave generator and, more particularly, to a circuit for generating a parabolic current of a satisfactory waveform where a complete continuity is ensured at the peak thereof without any level difference.
2. Description of the Prior Art
It is known that a color television receiver, a monitor or the like is equipped with a dynamic convergence circuit for automatically correcting any misconvergence so as to properly converge the beams emitted from three electron guns, and there is included a circuit for forming a parabolic current which is nonsymmetrical in waveform.
FIG. 6 is a schematic circuit block diagram of a conventional parabolic wave generator a, wherein a sawtooth wave transmitted from an unshown external circuit is processed in a squaring circuit b and then is supplied to one of amplifiers d and d' which are provided individually on two lines and are operated selectively via a switching circuit c. Thereafter the out of the amplifier is forwarded via another switching circuit e (interlocked with the first-mentioned switching circuit c) so that a parabolic wave is obtained from an output terminal f.
Since the amplifiers d and d' are so arranged that the gains thereof are controllable independently of each other, the parabolic wave can be shaped nonsymmetrical with respect to its peak by selectively setting the respective gains k, k' of the amplifiers d, d' at different values.
Regarding an equation y=a.multidot.t.sup.2 (where t denotes the time, and y denotes the amplitude having a relation of y=x.sup.2 (t) in which the original signal is expressed as x(t)) in the coordinate system with the origin O representing the peak of the parabolic wave, changing the gains k, k' corresponds to setting the coefficient a in the above equation to mutually different values in the range of t.gtoreq.0 and t&lt;0. (See the curve g represented by a broken line in the graph of FIG. 7.)
However, in the above circuit a of FIG. 6 where the squaring circuit b has an offset component, the parabolic wave fails to be shaped smoothly continuous in the vicinity of its peak at the switching time of the two signal lines by the switching circuits c and e, hence raising a problem that a level difference .DELTA.h is induced as represented by a solid-line curve h in the graph of FIG. 7.
The offset in the squaring circuit b is derived from the relative variation existing in the base-emitter voltages V.sub.BE of transistors constituting a differential pair.